runtime adapter: define each message kind's code once #665

Follow-up to #640.

The runtime adapter's framing, in tests/runtime-adapter/src/frame.rs, writes each message kind's code as a bare number twice: Kind::from_code maps a header's kind byte to a Kind, and Kind::code maps a Kind back to its byte. Adding or renumbering a kind means keeping both matches in step by hand.

Give each code a single definition that both directions use, for example:

• named constants for the codes;
• explicit discriminants on a #[repr(u8)] Kind, so that code is a cast and from_code matches the same values; or
• a small, maintained crate that derives the conversion from u8, as long as the guest-only build stays free of host-only dependencies, which tests/independent_build.rs checks.

Done when:

• each kind's code is written once in frame.rs;
• the table of kinds in tests/runtime-adapter/README.md still matches the codes;
• the adapter's checks pass in every configuration, as its README lists them.